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Background    Rickettsia raoultii  is a novel  Rickettsia  species recently isolated from  Dermacentor  ticks and classified within the spotted fever group (SFG). The inability of  R. raoultii  to spread within L929 cells suggests that this bacterium is unable to polymerize host cell actin, a property exhibited by all SFG rickettsiae except  R. peacocki . This result led us to investigate if RickA, the protein thought to generate actin nucleation, was expressed within this rickettsia species.    Methodology/Principal Findings  Amplification and sequencing of  R. raoultii rickA  showed that this gene encoded a putative 565 amino acid protein highly homologous to those found in other rickettsiae. Using immunofluorescence assays, we determined that the motility pattern (i.e. microcolonies or cell-to-cell spreading) of  R. raoultii  was different depending on the host cell line in which the bacteria replicated. In contrast, under the same experimental conditions,  R. conorii  shares the same phenotype both in L929 and in Vero cells. Transmission electron microscopy analysis of infected cells showed that non-motile bacteria were free in the cytosol instead of enclosed in a vacuole. Moreover, western-blot analysis demonstrated that the defect of  R. raoultii  actin-based motility within L929 cells was not related to lower expression of RickA.    Conclusion/Significance  These results, together with previously published data about  R. typhi , strongly suggest that another factor, apart from RickA, may be involved with be responsible for actin-based motility in bacteria from the  Rickettsia  genus.

RickA Expression Is Not Sufficient to Promote Actin-Based Motility of Rickettsia raoultii